Changes that take place in irreversible displays generally involve the revelation of indicia, which can range from a patch of color to text and pictures. The indicia can be revealed by chemical or physical agents that change themselves or that produce other changes in the displays. For example, opaque coloring agents can be rendered transparent to reveal underlying indicia, or similar agents can change from one color to another to indicate a change.
Chemical transformations in irreversible displays are sometimes used for security purposes to provide evidence of tampering or counterfeiting. U.S. Pat. No. 4,488,646 to McCorkle hides a warning message behind a solvent-sensitive blush coating to provide evidence of solvent tampering with letters, tickets, and other information-bearing constructions. Upon exposure to a wide range of aromatic or aliphatic solvents, the blush coating is transformed into a transparent state revealing the message. U.S. Pat. No. 4,903,991 to Wright discloses a document security system in which a latent image is developed by rupturing photoactive microcapsules to verify authenticity.
Mechanical transformations are more often used for interactive game pieces. The most common are scratch-off games in which an opaque coating is removed by abrasion to reveal a hidden indicium. Chang et al. in U.S. Pat. No. 5,431,452 separately position a latent image and a removable image-developing device on different portions of a substrate. The image-developing device contains a chromogenic composition that converts the latent image into a visible image.
Commonly assigned U.S. application Ser. No. 09/426,225 entitled "Irreversible Thin Film Display with Clearing Agent", which is hereby incorporated by reference, exploits features of thin metal films for temporarily obscuring indicia from view. Chemical clearing agents clear the metal films to display the indicia. The thin metal films can be cleared to reveal underlying indicia or the indicia can be formed by clearing the metal films in predetermined patterns.